In recent years, the growing awareness of the merits to use renewable energies, but also the high oil prices, have lead to a new approach in the use of liquid crystal materials. One main application of liquid crystal materials is their application in LCD screens. This very well-known technology is based on the construction of cells containing liquid crystals. However, the imperative use of dichroic filter polarization and absorbent filters in the construction of cells causes a substantial loss of light intensity and implies a high consumption of electricity.
In order to minimize this energy consumption, new technologies based on organic light emitting diodes (OLEDs) are currently used in the design of flat low-power displays. They are based on luminescent organic compounds that are introduced into devices in an amorphous state by spin coating or vacuum deposition. Their main advantages over LCDs are that their luminescence properties make the use of polarizers, backlight or filters unnecessary and that tailored flexible displays can be designed.
Three basic colors are needed in order to obtain every color of the visible light spectrum: red, green and blue. It is well known that many organic compounds, inorganic, or hybrid organic-inorganic compounds are able to deliver effectively in the green and blue area of the spectrum, but only very few compounds are able to deliver effectively in the 550-950 nm area (red and near infrared). Well known in the prior art are, for example, Lanthanide-based organometallic complexes, that are interesting due to their fine band of emission. However, their light absorption is weak, which is a major disadvantage for applications, as the photoluminescence intensity is proportional to the amount of adsorbed light. Furthermore, for the considered wavelength area (550-950 nm), the emission quantum yield of their luminescence is very low, with less than 0.1%. By contrast, the similarly well-known Platinum complexes, as described for example in WO2009/040551, have greater emission quantum yield, however, their production cost is very high.
It has been shown that Re based metal clusters show luminescence properties. Dorson, et al. (Dalton Trans., 2009, 1279-1299, which is incorporated by reference for the corresponding discussion) discloses a high yield and simple preparation method of hexahydroxo-complexes based on a Rhenium and Selen or Sulfur containing metal cluster. The cluster has four ligands of p-tertbutylpyridine (TBP) bound, and the modified octahedral cluster is suitable for further functionalizing on the two remaining hydroxyl groups with, for example, gallic acid derivatives or other carboxylic acids. The Re6 cluster derivative shows a high absorption in the UV region. The maximum of emission in the spectrum is at 720 nm.
It is well known in the field that many halides, chalcogenides, and chalcoh alogenides of Molybdenum or Tungsten contain octahedral clusters in which metallic atoms are held together by metal-metal bonds. The metallic octahedron is surrounded by eight face-capping and six terminal ligands to form a [M6Qi8Qa6]2-nano-sized unit (Q=chalcogen and/or halogen, i=inner, a=apical).
As shown by Kirakci, et al. (Z. Anorg. Allg. Chem. 2005, 631, 411, which is incorporated by reference for the corresponding discussion) and Long, et al. (J. Am. Chem. Soc. 1996, 118, 4603, which is incorporated by reference for the corresponding discussion) many routes afford soluble discrete [M6Qi8Xa6]2− units (X=halogen) that exhibit, either in the liquid or solid state, specific electronic, magnetic and photophysical properties related to the number of metallic electrons available for metal-metal bonds. It has been shown by various groups that they are, in particular, highly emissive in the red-NIR area with photoluminescence emission quantum yields up to 23% for Molybdenum based clusters and 39% for Tungsten based clusters (for Tungsten clusters, see for example Zietlow, Thomas C.; Nocera, Daniel G.; Gray, Harry B. Photophysics and electrochemistry of hexanuclear tungsten halide clusters. Inorganic Chemistry (1986), 25(9), 1351-3: [(n-C4H9)4N]2 W6I8I6 quantum yield 0.39; λmax=698 nm; luminescence ranging from 550 to 950 nm). They, furthermore, display excited state lifetimes ranging from the tenth to the hundredth microseconds and undergo facile ground and excited state electron transfer by electro-generated luminescence. Owing to the stronger covalent nature of the M-Qi bond compared to the M-Xa one, halogen apical atoms can be replaced by inorganic or organic ligands without any alteration of the (M6Qi8)m+ core leading to functional building blocks usable for the design of supramolecular architectures, polymeric frameworks or nanomaterials with unique properties. Many examples of hexasubstituted [M6Qi8La6]x− units (L=ligand) have been reported so far. However, because of the nanometric size of the (M6Qi8)m+ core and the sensitivity of the [M6Qi8La6]x− units to air and moisture, their functionalization and use as liquid crystalline components in devices remain challenges. It requires air stable hybrid compounds with, on one hand, self organization abilities and, on the other hand, fluidity in order to correct automatically the positioning errors that can occur during the assembly process.
Thus, a need exists to address the above challenges.